Spatial light modulators have a wide range of practical uses, such as video display or projection. They are also employed in correcting phase aberrations in a light beam caused by atmospheric effects. Phase aberrations are readily determined using well-known optical techniques, from which corrective phase. A spatial light modulator can correct such phase aberrations by imposing respective phase shifts for different picture elements (pixels) in an image plane. Such a spatial light modulator can be fabricated in a silicon-on-insulator structure as disclosed in U.S. Pat. No. 5,844,711. In that patent, a planar reflective layer containing a conductive thin film is deformed toward another planar layer containing its own conductive thin film by a voltage (e.g., 40 volts) applied between the two conductive thin films. In this device, the deformation is a quarter wavelength of the light for a half wavelength phase shift. Each pixel then has either a half wavelength phase correction or zero phase correction, depending upon the control voltage applied to that pixel. As the deformation draws the two conductive thin films closer to one another, the electrostatic attractive force increases until it exceeds the opposing mechanical force of elastic deformation. At this point, the one layer snaps hard against the opposing layer and remains at a maximum deflection, as long as a very small remnant voltage (e.g., 10 volts) is maintained. The pixel is returned to its original undeformed condition by removing the remnant voltage. Thus, once a maximum deflection has been made, the applied voltage may be removed, except for a small remnant, while retaining the phase correction. In the proposed device this feature is exploited to facilitate row-by-row application of a desired spatial phase correction pattern. Of course, such a spatial light modulator is of limited accuracy in that it is incapable of providing non-zero phase corrections of less than a half wavelength, and each pixel correction is a binary choice: ON (half wavelength phase shift) and OFF (zero phase shift). Thus, it may be said to have only one-bit accuracy.
To date, it has not seemed practical to improve accuracy of such devices beyond a single bit. One way of increasing accuracy would seem to be applying different voltage levels to attain different amounts of deformation. For example, four bits of accuracy could be attained by assigning four different voltage levels to choose among for the applied voltage. The main problem with this approach is that the voltage levels must be limited in order to keep the deformation less than the maximum or critical deformation (at which the surface snaps into the maximum deformation), in order to stay within a range in which different voltage levels produce different amounts of deformation. Unfortunately, this requires that the applied voltage remain on, which prevents row-by-row imaging of a desired spatial phase correction pattern. The way around this problem is to provide individual conductors for each pixel, but this greatly increases the complexity of the device, leading to congestion of many conductive paths on the substrate surface. One way of alleviating such congestion is to place the pixel control circuitry on the back of the substrate with metal contacts established through the entire thickness of the device. However, such an approach is limited by the small pixel area and greatly increases cost and device complexity.
Even if such a multi-voltage level approach were practical, it would suffer from a more fundamental limitation arising from variations in device characteristics across the surface of the substrate. Process variations are unavoidable and necessarily lead to different thin film thicknesses in different areas of the substrate. Variations in thickness lead to variations in mechanical stiffness across the substrate. As a result, the amount of deformation (and therefore the phase shift) produced at a particular applied voltage level will be different in different areas of the substrate. Such variations reduce the pixel-to-pixel accuracy of the spatial light modulator. Solving this problem requires a significant tightening of fabrication process controls, which increases the cost even further.
Therefore, it has seemed too costly or impractical to provide a spatial light modulator having more than single bit accuracy.
A spatial light modulator having n bits of phase resolution has n stacked layers of deformable electrode pairs supporting a deformable reflective surface, each electrode pair included in upper and lower stacked elongate ribbon conductors extending in respective orthogonal directions, each coincidence of an upper and lower electrode pair defining a pixel within the respective stacked layer, each electrode pair deforming to a maximum deformation in response to an applied write voltage across the pair so as to produce at the reflective layer a phase correction of the corresponding pixel proportional to the corresponding deformation, and wherein the maximum deformation of the pixels in each stacked layer is the same, and wherein the maximum deformation of successive layers decreases in ascending order of the stacked layers.
The maximum deformation can decrease with successive ones of the n stacked layers by ascending negative powers of 2, wherein a phase correction can be written to an individual one of the pixels with n-bit accuracy. The lower ribbon conductors typically are mutually parallel in a first direction and coplanar in a first plane, and the upper ribbon conductors typically are mutually parallel in a second direction orthogonal to the first direction and coplanar in a second plane vertically displaced from the first plane.
Each one of the layers typically includes elongate ribs between respective ones of the lower elongate ribbon conductors that are within the one layer, the ribs providing support to the upper elongate conductors to form therebetween a compression gap defining the maximum deformation, and a deformable membrane resting on top of the ribs and supporting the upper elongate ribbon conductors that are within the one layer. Typically, a table supports the lower ribbon conductors of the bottom one of the layers and an insulating layer between the table and the lower ribbon conductors of the bottom one of the layers. Typically, an insulating layer lies between the deformable membrane and the upper ribbon conductors and an insulating layer covers the lower elongate ribbon conductors. Another insulating layer typically covers the upper elongate ribbon conductors. The layers can be separated by pedestals, each pedestal extending between the upper elongate ribbon conductors and the table of the layer immediately above the one layer.